In order to enable mobility in cellular mobile communications systems, a key feature is the ability to perform a handover from one cell currently serving the mobile terminal to a new cell. The variety of handover techniques available today can be sorted into a variety of different classes. For example, a ‘soft handover’ comprises that a mobile terminal is connected for some time span during the handover to two base stations in parallel, while in a ‘hard handover’ the mobile terminal disconnects from the old base station before connecting to the new base station. A further classification sorts handover techniques into ‘network controlled handover’, ‘mobile assisted handover’ and ‘mobile controlled handover’.
The 3th Generation Partnership Project (3GPP) is a standardization body currently working on a system concept termed ‘Long Term Evolution (LTE) and System Architecture Evolution’ (SAE). The handover technique that will presumably be used in the 3GPP LTE/SAE system is a mobile assisted hard handover, wherein the mobile assistance comprises that the mobile terminal or User Equipment (UE) performs downlink radio signal measurements, while the network will make the handover decision. The architecture of the 3GPP LTE/SAE system will be flat, i.e. the radio base stations termed ‘evolved Node Bs’ (eNBs) are directly connected with each other in the radio access network E-UTRAN (Evolved UMTS Radio Access Network) and are also directly connected to the core network. Therefore, there is no central controller such as the Base Station Controller (BSC) in a GSM system or Radio Network Controller (RNC) in a WCDMA or UTRAN system, in which the handover algorithm could be located. Instead, the handover decisions will be performed by the eNBs, specifically by the cell currently serving the mobile terminal.
In LTE, cells can be identified by unique cell identifiers as well as non-unique cell identifiers. A unique cell identifier (“Cell Identity”) is uniquely identifying a cell at least within one mobile communications network (Public Land Mobile Network, PLMN). A non-unique cell identifier (“Physical Cell Identity”) is in general not unique in a network, and may even not be unique in a local environment, (i.e. around a particular base station). However, the non-unique identifier when broadcasted, for example, from a candidate cell for handover can be detected by a UE relatively easy. For example, in LTE it is planned that 504 physical cell identities will be provided, wherein each identity is associated with a specific physical fingerprint of reference symbols. The unique cell identifier, on the other hand, is broadcasted as system information which is regularly but not continuously transmitted. The UE therefore needs to wait for the next time the unique cell identifier is broadcasted before it can be received and decoded.
The eNB maintains for each served cell a neighbour cell list indicating neighbouring cells. This list associates the non-unique identifier of a neighbouring cell and/or the unique cell identifier for the cell with connectivity information related to how to reach the eNB serving that cell. In many cases it will be sufficient that the UE reports the non-unique cell identifier of the candidate cell for handover to the serving eNB: When the non-unique cell identifier is known and locally unique, the eNB can connect to the base station of the candidate cell based on the neighbour cell list. If a non-unique cell identifier is provided which is unknown or is in fact non-unique in the neighbour cell list, the serving eNB requests the UE to provide the unique cell identifier for the candidate cell. Then the UE has to receive the system information from the candidate cell, has to decode the unique cell identifier therefrom and has to transmit the decoded unique cell identifier to the serving eNB.
In certain situations the mobile terminal will have no time to decode the unique cell identifier. Consider, for example, a case in which the mobile terminal is currently involved in a Voice-over-IP (VoIP) communication, which comprises receiving small data packets in regular intervals. The periodical data reception may prevent that the terminal can receive system information from a neighbouring cell in case of overlapping time slots (assuming that there is only one receiver chain available). In another exemplary situation, uplink data is acknowledged from the network, i.e. acknowledgements have regularly to be received which may also prevent receiving system information from a neighbouring cell. In principle, any service with too little discontinuous reception, either due to an ongoing reception of downlink data or of feedback to uplink data, may prevent receiving the unique cell identity. Thus, if any data is to be received, currently it is envisaged that the terminal may skip the provision of the unique cell identifier to the serving cell. However, this then prevents handover to the corresponding cell.
This situation will become particularly problematic, for example, in the field of Closed Subscriber Groups (CSGs). In such an environment, typically not much effort will be spent for network planning. For example, CSG cells may have assigned a non-unique cell identifier at random from a reserved set of non-unique cell identifiers. Then it will happen relatively often that a non-unique cell identifier is also locally non-unique. It is envisaged that the handover mechanism for handover to a CSG cell comprises a mandatory determination of the unique cell identifier of the cell (in case of CSG cells in LTE, apart from the Cell Identity a second kind of unique identifier is handled which indicates a CSG Identity; this CSG Identity is used for authorization purposes by the mobile terminal and therefore needs to be determined—however, it is the Cell Identity that may be mandatory in a measurement report to the serving cell). However, in the frequent case that the mobile terminal is involved in a conversational service, streaming service or similar service with insufficient discontinuous reception, a handover to such cell is prevented, as the terminal is unable to decode the unique cell identifier thereof. This problem poses a limit to the applicability of certain network scenarios such as CSGs.
WO 2009/065053 A2 relates to using identifiers to establish communication, wherein confusion resulting from assigning the same node identifier to multiple nodes is resolved through the use of confusion detection techniques and the use of unique identifiers for the nodes. In some aspects an access point and/or an access terminal may perform operations relating to detecting confusion and/or providing a unique identifier to resolve confusion.
Document XP050340920, Nokia Siemens Networks et al.: “CSG enhanced mobility requirements” relates to Closed Subscriber Group (CSG) enhanced mobility requirements and in particular to an analysis of the CSG connected mode mobility requirements. In most scenarios the connected mode mobility could be provided with already existing REL8 SIB1 reading mechanism. Only cases when it may not provide excellent user perception is during VoIP type of applications as then the DRX periods are more seldom. But even for those scenarios a couple of solutions are provided, i.e. User Equipment capability of reading neighbour cell SIBs and/or User Equipment initiated transmission pauses.